This invention relates to a process and a means for manufacturing cast iron containing vermicular graphite.
Cast iron containing vermicular graphite (referred to herein as "GGV") is used as a construction material frequently between cast iron containing lamelle graphite (GGL) and cast iron containing nodular graphite (GGG). This construction material is superior to the GGL material in view of its special mechanical qualities, such as it high tensile strength, sturdiness and elasticity module. In comparison to the GGG construction material, cast iron containing vermicular graphite possesses higher heat conductivity and better casting ability.
The demand for the GGV construction material has risen dramatically during the last years. However, it has been difficult to master a precise, reproducible process for manufacturing GGV in order to keep pace with rising demand. Because of this, many plants were forced to abstain from the manufacture of GGV.
In DE-OS No. 24 58 033 and its corresponding U.S. Pat. No. 3,666,449, which are incorporated herein by reference, a process is described in which a starting melt is pre-treated with magnesium until the sulphur content falls below 0.01% by weight of S, and in which the time for adding rare earth metals to the melt is coordinated with the Mg treatment so that nodular graphite is not produced.
In DE-OS No. 24 58 033 a process is also described in which the starting melt is treated with Mg prior to adding rare earth metals, such as a cerium alloy. In this process as well, the amount of Mg which is added is controlled in such a way that no more than 0.01% by weight of S remains and so that nodular graphite is not produced.
It is the task of the present invention to improve the known iron making processes in a way so that cast iron with vermicular graphite can be produced in a rapid, precise and reproducible manner.